Fibrous filter media are produced in a wide variety of forms and are employed in many diverse applications. For many applications, these media must be able to perform at high efficiency over an extended period of time when exposed to high temperatures and corrosive fluids (e.g., in the processing of aramid fibers, a slurry of the fibers in hot concentrated sulfuric acid must be filtered to effectively separate the fibers from the acid by a filter medium which must maintain its integrity in service over an extended period of time). Thus, it is very important that both the fibers and the binder resin in the fibrous media be capable of withstanding this type of potentially highly destructive environment over the useful life of the filter.
When the environment requires the use of strong acids or bases and/or requires continuous use at temperatures in excess of 350.degree. F, the only resin types that can adequately meet these stringent requirements are fluorocarbon polymers. Glass fibers are often used in such media since, while they are susceptible to attack in strongly acidic or alkaline environments, their integrity can generally be maintained over a reasonable time. To the extent that their integrity could be extended, this would be desirable.
Past efforts to impregnate fibrous structures with fluorocarbon resins have been only partially successful. The prior methods generally involve a dip coat post impregnation technique. This technique typically leads to a non-uniform sandwich-type structure with the resin coating primarily on the top and bottom surfaces of the fibrous structure with limited or no bonding throughout the body of the structure resulting in limited structural integrity.
The subject invention is directed to a method for preparing porous, fibrous structures which overcomes the problem of non-uniform dispersion of the fluorocarbon resin and to the resulting structures which have a substantially uniform coating of the fluorocarbon resin throughout the structure. It is further directed to extending the life of fibrous glass structures.